The invention relates generally to methods for applying an underfill between an electronic device and a substrate.
It is typical for an electronic device, such as a flip chip, chip scale package (CSP), ball grid array (BGA) or package on package assembly (PoP) to include a pattern of solder bumps that, during mounting, are registered with pads on a substrate, or joined using another type of interconnect technology such as copper pillars or other types of thermal compression bonding interconnects. The substrate can be a printed circuit board, electronic chip or wafer, for example. The solder is reflowed by heating and, following solidification, solder joints connect the electronic device and the substrate. Underfill, which may be an epoxy, may be used to fill the open space between the electronic device and the substrate that remains between the reflowed solder balls. The underfill protects the solder joints against various adverse environmental factors, redistributes mechanical stresses due to shock, and prevents the solder joints from moving under strain during thermal cycles.
In the process of underfilling, voids may be formed due to the following reasons, but not limited to, uneven surface topography in the gap between the electronic device and substrate, fluid flow rate race conditions as underfill flows around a solder connection, different wetability conditions on the substrate, air in the underfill, or air induced from the dispensing process. Because the voids are unfilled by underfill, unsupported solder joints adjacent to voids may not be adequately protected against cold flow when exposed to strain from thermal expansion during operation or to mechanical shock caused by dropping the assembled end product, such as a cell phone, that includes the underfilled electronic device. Voids at solder joints prevent the solder bump from being in held in a state of hydrostatic compression and strain restraint, which may increase solder joint fatigue and thereby increase the probability of solder joint cracking.
Therefore, improved methods are needed for applying an underfill that reduces the probability of forming voids in the underfill.